1. Field of the Invention
The present invention relates to a device for optically tracking information, and more particularly to an information tracking device for tracking information which is recorded on an optical disc in the form of a track.
2. Description of the Prior Art
An optical system for tracking video or audio information which is recorded on a rotating disc in the form of a track, to optically reproduce the recorded information is disclosed in, for example, a Japanese patent application Laid-Open No. 93222/1977. In addition to such an optical system, an optical system shown in FIG. 1 can be used to carry out tracking. When a tracking signal is formed using the optical system shown in FIG. 1, a photodetector and a signal processor can be used which are disclosed in the above-referred Japanese patent application. FIG. 2 shows the above-mentioned photodetector and signal processor.
Referring to FIG. 1, light emitted from a laser light source 1 passes through a coupling lens 2, a beam splitter (that is, a polarizing prism) 3, a galvanometer mirror 4, a quarter-wave plate 5 and an object lens 6, to form a light spot 7 on an array of information pits (that is, a track 9) which is formed on the surface of a disc 8. Light which has been diffracted by the information pits and reflected by the disc 8, passes through the lens 6, quarter-wave plate 5 and mirror 4, and is then reflected by the beam splitter 3 due to the function of the quarter-wave plate 5, to be received by a photodetector 10.
In the above-mentioned optical system, the quantity of light returning to the laser light source 1 can be reduced without being affected by the reflection factor at the mirror 4. This is because the mirror 4 is disposed between the polarizing prism 3 and quarter-wave plate 5. This fact will be explained below in detail. Now, let us assume that the polarizing prism 3 transmits P polarized light and interrupts S polarized light.
Further, let us assume, for simplicity's sake, that the transmission factor of the P polarized light having passed through the polarizing prism 3 is 1, the reflection factor of the S polarized light at the polarizing prism 3 is 1, the retardation due to reflection at the disc 8 is zero, and respective reflection factors of the P polarized light and S polarized light at the disc 8 are both equal to 1.
When the plane of polarization of light having passed through the polarizing prism 3 makes 45.degree. with X- and Y-axes of a rectangular coordinate system and the reflection factors of the P polarized light and S polarized light at the mirror 4 are expressed by a and b, respectively, X- and Y-components E.sub.1X and E.sub.1Y of light having passed through the prism 3 and having been reflected by the mirror 4 are given by the following equations: EQU E.sub.1X =aP sin .omega.t, E.sub.1Y =aP sin .omega.t (1)
where P indicates the output power of light emitted from the laser light source 1, and .omega. the angular frequency of light.
That is, the light having been reflected by the mirror 4 is linearly-polarized light. When this light passes through the quarter-wave plate 5, only the Y-component E.sub.1Y is subjected to a 90.degree. phase shift. Thus, the light having passed through the quarter-wave plate 5 has X- and Y-components E.sub.2X and E.sub.2Y as follows: ##EQU1## That is, the light having passed through the quarter-wave plate 5 is circularly-polarized light.
Further, X- and Y-components E.sub.3X and E.sub.3Y of light reflected from the disc 8 are given by the following equations: ##EQU2## That is, the light reflected from the disc 8 is circularly-polarized light which is opposite in the direction of rotation of electric field vector to the circularly-polarized light incident on the disc 8. The light having been reflected from the disc 8 passes through the quarter-wave plate 5, and light having passed through the quarter-wave plate 5 has X- and Y-components E.sub.4X and E.sub.4Y as follows: ##EQU3## That is, the reflected light having passed through the quarter-wave plate 5 is perfect, linearly-polarized light, and the plane of polarization thereof is rotated through 90.degree. as compared with the plane of polarization of the incident light from the mirror 4. Thus, the quantity of light capable of returning to the laser light source 1 is theoretically equal to zero.
As mentioned above, when the mirror 4 is disposed between the polarizing prism 3 and quarter-wave plate 5, the plane of polarization of light which is incident on the polarizing prism 3 after having been reflected from the disc 8, is rotated through 90.degree., without being affected by the reflection factor at the mirror 4, and thus this light becomes perfect, S polarized light.
In contrast to the above-mentioned, when the mirror 4 is disposed between the quarter-wave plate 5 and disc 8, light incident on the polarizing prism 3 after having been reflected from the disc 8 is affected by a difference between respective reflection factors of the P polarized light and S polarized light at the mirror 4, and therefore the plane of polarization of this light is not subjected to 90.degree. of rotation, that is, the light cannot become perfect, S polarized light. Thus, the light contains a P polarized light component, and therefore it is inevitable that some light quantity returns to the laser light source.
In an optical disc device in which a semiconductor laser is used as a light source, even a very small quantity of light which is returned from an optical disc to a laser resonator, disturbs the oscillating state of the laser, and causes noise. Accordingly, it is specifically important in such a device to reduce the quantity of light returning to the laser.
Incidentally, in FIG. 1, reference numeral 11 designates a motor for rotating the disc 8, and 12 a spindle of the motor 11.
Referring now to FIG. 2, the origin of a rectangular coordinate system at the photodetector 10 is placed on the optical axis of the optical system shown in FIG. 1, and X- and Y-axes of the coordinate system are set in parallel with and perpendicularly to the direction of the track on the disc, respectively. The photodetector 10 is made up of four photodetectors 11, 13, 12 and 14 situated respectively at the first, second, third and fourth quadrants. These photodetectors 11, 13, 12 and 14 deliver output signals I.sub.11, I.sub.13, I.sub.12 and I.sub.14, respectively. A difference signal DF expressed by an equation DF=(I.sub.11 +I.sub.12)-(I.sub.13 +I.sub.14) (hereinafter referred to as "DF signal") is formed by adders 15 and 16 and a subtracter 17. A summation signal RF expressed by an equation RF=(I.sub.11 +I.sub.12)+(I.sub.13 +I.sub.14) (hereinafter referred to as "RF signal") is formed by adders 15, 16 and 18. The RF signal 24 thus formed is applied to a phase shifter 19 to shift the phase of the RF signal by 90.degree.. The output of the phase shifter 19 and the DF signal 23 are applied to a multiplier 20, the output of which is applied to a low-pass filter 21 to obtain a tracking signal 22. In such a conventional device, the combination of the multiplier 20 and low-pass filter 21 is used to obtain the tracking signal. Accordingly, the device has the following problems:
(i) The tracking error detecting sensitivity is low, that is, the so-called tracking signal 22 which is produced when the light spot 7 deviates from the center line of the information track 9, is outputted at a low level.
(ii) The tracking offset which is produced when the disc 8 or galvanometer mirror 4 is inclined, is large.